Active coil to shift a volume of uniform magnetic field

ABSTRACT

Gradient coils are operated to acquire magnetic resonance (MR) signals encoding a first MRI image over a first region inside a main magnet of the MRI system in which at least a portion of a subject is placed, the first region being located within a volume of uniform magnetic field with inhomogeneity below a defined threshold. An active coil is energized to shift the volume of uniform magnetic field such that a second region inside the main magnet of the MRI system is located within the shifted volume of uniform magnetic field, at least a portion of the second region being located outside of the volume of uniform magnetic field before the volume of uniform magnetic field has been shifted. The gradient coil is operated to acquire MR signals encoding a second MRI image over the second region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/899,799, filed Dec. 18, 2015, which is a U.S. National PhaseApplication of International Patent Application No. PCT/IB2015/055366filed Jul. 15, 2015, the contents of both are hereby incorporated byreference:

BACKGROUND

The present disclosure relates to magnetic resonance imaging.

SUMMARY

In one aspect, some implementations provide a method for operating amagnetic resonance imaging (MRI) system, the method including operatinggradient coils to acquire magnetic resonance (MR) signals encoding afirst MRI image over a first region inside a main magnet of the MRIsystem in which at least a portion of a subject is placed, the firstregion being located within a volume of magnetic field where a fieldinhomogeneity is below a defined threshold; energizing an active coil toshift the volume of magnetic field such that a second region inside themain magnet of the MRI system is located within the shifted volume ofmagnetic field, at least a portion of the second region being locatedoutside of the volume of magnetic field before the volume of magneticfield has been shifted and at least a portion of the first region beinglocated outside of the shifted volume of magnetic field after the volumeof magnetic field has been shifted; and operating the gradient coils toacquire MR signals encoding a second MRI image over the second region.

Implementations may include the following features. For example,energizing the active coil may include energizing the active coil whileoperating the gradient coils to acquire the second MRI image. In anotherexample, energizing the active coil may include energizing the activecoil prior to operating the gradient coils to acquire the second MRIimage. In yet another example, energizing the active coil includesdriving the active coil with at least 2 amperes of electrical current.In still another example, energizing the active coil may include coolingthe active coil by running liquid coolant to the active coil.

Some implementations may additionally include operating a shim coil tocompensate susceptibility differences inside the portion of the subjectlocated within the shifted volume of magnetic field with inhomogeneitybelow the defined threshold. In these implementations, operating theshim coil may further include operating the shim coil while operatingthe gradient coils to acquire MR signals encoding the second MRI image.

In another aspect, some implementations provide a magnetic resonanceimaging (MRI) system, including a housing having a bore in which atleast a portion of a subject to be imaged is placed; a main magnetaccommodated by the housing and configured to generate a volume ofmagnetic field with inhomogeneity below a defined threshold to form anMRI image over a region located within the volume of magnetic field; anactive coil that when energized causes the volume of magnetic field toshift in location and transform in shape; pulse generating coils togenerate and apply radio frequency (RF) pulses in sequence to scan theportion of the subject; gradient coils to provide perturbations to thevolume of magnetic field such that MRI signals encoding an MRI image areacquired in response to the applied RF pulses; and a control unitcoupled to the MRI system and configured to: operate the gradient coilsto acquire MR signals encoding a first MRI image over a first regioninside the main magnet in which the portion of the subject is placed,the first region being located within the volume of magnetic field inwhich a field inhomogeneity is below the defined threshold to form thefirst MRI image; energize the active coil to shift the volume ofmagnetic field with inhomogeneity below the defined threshold such thata second region inside the main magnet of the MRI system is locatedwithin the shifted volume of magnetic field in which the fieldinhomogeneity is below the defined threshold, at least a portion of thesecond region being located outside of the volume of magnetic fieldbefore the volume of magnetic field has been shifted and at least aportion of the first region being located outside of the shifted volumeof magnetic field after the volume of magnetic field has been shifted;and operate the gradient coils to acquire MR signals encoding a secondMRI image over the second region being located within the shifted volumeof magnetic field in which the field inhomogeneity is below the definedthreshold to form the second MRI image.

Implementations may include one or more of the following features. Theactive coil and the gradient coils may be integrated into one mechanicalassembly. The integrated coil assembly may include one liquid coolingsystem for both the active coil and the gradient coils. The active coilmay be constructed as a removable module configured to be mounted alongwith a gradient assembly that houses the gradient coils. The active coiland the gradient coil may be separately cooled by respective liquidcooling systems. The respective cooling system may incorporate areservoir holding liquid helium or liquid nitrogen.

The active coil may be constructed on an RF coil assembly configured tobe mounted inside the gradient coils. The active coil and the gradientcoils may be separately cooled by respective liquid cooling systems. Thecooling system may incorporate a reservoir holding liquid helium orliquid nitrogen.

Some implementations may include a group of shimming coils placed aroundthe housing and configured to compensate susceptibility differencesinside the portion of the subject located within the volume of magneticfield. The control unit may be further configured to energize the activecoil to shift the volume of magnetic field wherein variations inmagnetic field strength within the volume of magnetic field are belowthe defined threshold. The control unit may be further configured toenergize the active coil to shift the volume of magnetic field wherein afree induction decay (FID) signal from the volume of magnetic field hasa spectral width that is below the defined threshold.

The details of one or more aspects of the subject matter described inthis specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective view of an example of a magnetic resonanceimaging (MRI) system with a solenoid magnet where an active coil isprovided to shift a volume of uniform magnetic field inside the solenoidmagnet.

FIG. 1B shows a cross-sectional illustration of the example of amagnetic resonance imaging (MRI) system where the active coil isprovided to shift the volume of uniform magnetic field.

FIG. 2 illustrates an example of shifting the volume of uniform magneticfield inside the solenoid magnet of the MRI system.

FIG. 3 shows an example of operating the MRI system with the active coilto shift the volume of uniform magnetic field when imaging a subject.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

According to selected embodiments of the present disclosure, a magneticresonance imaging system can include an active coil, for example,integrated with gradient coils as one mechanical assembly, to functionas a location-specific static field shim coil which, when activated,moves the volume of uniform magnetic field within the solenoid magnetsuch that the imaging region where main field inhomogeneity is below adefined threshold is shifted. The active coil is different from the mainfield shimming coils that are configured to compensate forsusceptibility differences caused by air-tissue interfaces, implanteddevices, etc. Main field shimming coils generally cannot shift aside avolume of uniform magnetic field suitable for forming an MRI image withsufficient quality. The active coil as disclosed in some implementationsmay incorporate higher amperage current than those used for the shiftingoperation. In some instances, the active coil may include active coolingusing coolants such as liquid nitrogen or liquid helium. The active coiloperates differently than the gradient coil in that, once the activecoil is activated, the shifted region of uniform magnetic field staysstatic whereas turning on the gradient coil initiates a dynamicfluctuation to the main magnetic field such that MRI signals encodingmagnetization signals from various portions of the subject can beacquired, based on which an image reconstruction can be performed toyield an MRI image.

Some implementations may allow an operator, such as a clinician, toshift the image region from an initial region that covers, for example,the head area, to a different region that includes, for example, theneck area. More specifically, the patient may be placed in the mainmagnet and a first image is obtained from the head area. Afterdetermining that the neck area warrants further investigation, theoperator may activate the active coil to shift the region of uniformmagnetic field from the initial region to the different region thatincludes the neck area. When the shift is in place, the operator mayobtain an MRI image of the neck area. This shifting approach may beadvantageous for smaller magnets, in which it is less practical to movethe patient's neck inward due to limitations in magnet size and shape.

FIGS. 1A-1B show a perspective view and a cross-sectional view of anexample of a magnetic resonance imaging (MRI) system 100 in which asolenoid magnet 105 is provided in a cylindrical shape with an innerbore 101. Coil assembly 107, including transmit coil 106 and gradientcoil 104, is provided within solenoid magnet 105. Coil assembly 107 maygenerally be shaped as an annular structure and housed within the innerbore of solenoid magnet 105. In some implementations, annular coilassembly 107 only includes gradient coil 104. Gradient coil 104generally provides field gradients in more than one directions, such as,for example, all three orthogonal spatial directions. Thus, gradientcoil 104 may refer to three sets of coils, each configured to generatefield fluctuations in a respective direction for the main field in theinner bore of the solenoid magnet 105. Such field fluctuations may causemagnetizations from various spatial locations to experience precessionsat different frequencies, enabling encoding of spatial information ofthe magnetizations through RF excitation pulses.

In these implementations, annular coil assembly does not includetransmit coil 106 or any receiver coil. For these implementations,radio-frequency (RF) excitation pulses are, for example, transmitted bylocal coils for imaging the head region 102 of patient 103. In oneinstance, a head coil in a birdcage configuration is used for bothtransmitting RF excitation pulses and receiving MR signals for imagingthe subject. In another instance, a surface coil is used fortransmitting an RF excitation pulse into the subject and a phased arraycoil configuration is used for receiving MR signals in response.

In some implementations, shimming coils 109 are housed within thecylindrical walls of solenoid magnet 105. Shimming coils 109 are poweredby a group of power amplifiers. In some cases, the power amplifiers arehoused in a control room and are connected to shimming coils 109 toprovide shimming of the magnetic field within inner bore 101. In drivingshimming coils 109, power amplifiers may be controlled by a control unitthat generally includes one or more processors as well as programminglogic to configure the power amplifiers. In some instances, the controlunit is housed in a control room separate from the solenoid magnet 105of the MRI system 100. The driving current for shimming coils 109 may bein the range of hundreds of miliamperes and generally may not exceed 1ampere. Further, shimming coils 109 may not require active cooling usingcirculating coolant. In these implementations, an array of shimmingcoils can be used to provide adjustment to the field strength within theinner bore 101 such that the magnet field within the inner bore 101becomes more homogenous.

The embodiments provided herein may be adapted for intraoperative MRI,and MRI systems for use in an emergency room setting. Such MRI systemsmay include a smaller and more compact bore size magnet compared to themagnets from conventional whole body scanners. One consequence of asmaller bore magnet is that, the volume of uniform magnetic fieldsuitable for imaging (e.g., with field inhomogeneity below a definedthreshold) may not cover all areas of interest. As discussed in furtherdetail below, the defined threshold may refer to a variation of magneticfield over a spatial length, or a spectral width of a free-inductionsignal (FID) emitting from the particular volume. For example, while thehead region of a subject may be inside the volume of uniform magneticfield, the neck region of the subject may be not. Yet, the bore sizelimitation may not allow the subject to be moved further inside theinner bore of the magnet. Some implementations provide an active coil110 that, once activated, causes the volume of uniform magnetic field toshift in order to accommodate, for example, imaging the neck region ofthe subject when, for example, it is impractical to move the subjectfurther inside the bore of the magnet. In other words, active coil 110can provide a location specific static shift of the uniform magneticfield suitable for imaging.

For context, the main magnet of MRI system 100 generates a highlyuniform static magnetic field over a certain volume for imagingpurposes. Small static field variations on the order ofparts-per-million (ppm) can be tolerated; however, it is not possible togenerate MR data in locations where the main field deviates too greatly,for example, over hundreds of parts per million (ppms) over a 20-cmdiameter spherical volume. The region of uniformity, also referred to asthe uniform magnetic field or main field homogeneity, is typicallycharacterized by the maximum static field deviation over a certainspherical volume. For example, 40 ppm over a 25-cm diameter sphericalvolume (DSV) would represent a maximum ΔB0=B0_(max)−B0_(min)=20 μT fielddeviation at a static field of B0=0.5 T. The main magnet is designed toachieve a specific homogeneity (that is, the main magnet is designed tohave an inhomogeneity below the threshold); however, the actualhomogeneity at the installation site may be affected by material in oraround the MRI scanner. At the time of installation, passive and/oractive shims may be applied to improve the homogeneity so that it meetsthe specific homogeneity the main magnet is designed to achieve beforesubjects are placed in the inner bore 101. When a subject (i.e. a humanhead) is inserted into the MRI scanner, the tissue and any implantabledevices in the subject may also affect the homogeneity of the imagingvolume and the homogeneity is again typically improved through fineadjustment of active shim coils, such as for example, through shimmingcoils 109, so that the specific homogeneity is met. To quantify mainfield homogeneity, some implementations may measure, for example, thespectral width of the free induction decay (FID) signal from the regionof interest. In this measure, field homogeneity may hinge on thespectral width of the FID signal to be below a defined threshold. Morespecifically, if the spectral width of the FID signal is satisfactorilynarrow for the desired imaging application, for example, below a definedspectral width value, shimming may be deemed satisfactory. Otherwise,additional shimming may be performed to further reduce the spectralwidth of the FID signal.

In this context, for a whole body MRI scanner, the bore is wide enoughany part of a human can be moved to within the volume of uniformmagnetic field. In such systems, the subject can be moved so that theanatomy interest is located in the center of the region of uniformity.For a smaller-bore MRI scanner designed to image the head, geometricconstraints limit what parts of the body can be moved into the volume ofuniform magnetic field. For example, the geometry of a head-only MRIsystem could be such that an average human could be positioned so thatthe region of uniformity generally extends only to the bottom of thechin and for regions further out, the inhomogeneity of the main fieldmay exceed a defined threshold and does not permit MR imaging with thedesired quality. The decay in image quality may manifest as poorsignal-to-noise ratio, low peak value of the free induction decay (FID)signal, etc. Generally, imaging anatomy farther down the neck or spinefor such a person may not be feasible because such regions cannot beplaced physically further into the inner bore 101 of scanner.

As noted above, active coil 110, when energized, can provide a locationspecific static shift of the homogenous region. In this example, activecoil 110 could be energized during collection of MR signals fromlocations in the neck and spine where the main field might otherwise betoo inhomogeneous for acquiring MR signals of decent quality. Activecoil 110 may, in general, change the shape and location of the region ofuniformity such that the volume of uniform magnetic field is shiftedfrom the head area to enclose parts of the neck and spine.

FIG. 2 illustrates an example of shifting the volume of uniform magneticfield inside the solenoid magnet of the MRI system 100. Initially, asubject's head area 202 and neck area 204 are inserted into inner bore101 of MRI system 100 for an imaging session. A first MRI image may beformed for the head area 202 with a portion of neck area 204. Asillustrated, the volume of uniform magnetic field (i.e., regions ofuniformity 212 when coil off) does not extend from the head area 202 tofully cover the neck area 204. Yet, the shape and the size of inner bore101 prevents the subject to be further inserted, as noted earlier. Inthis illustration, the clinician can, through a control panel on MRIsystem 100, energize active coil 110 to shift the volume of uniformmagnetic field toward the neck area 204 so that the shifted volume ofuniform magnetic field (i.e., region of uniformity with coil on 214)fully covers neck area 204. Once the uniform magnetic field covers neckarea 204 such that main field inhomogeneity within neck area 204 fallsunder a defined threshold value, the clinician can initiate, through thecontrol panel on MRI system 100, a normal scan using gradient coil 104,RF coil 106, and shim coil 109 to obtain a desired image of neck area204.

In some instances, active coil 110 could remain energized for the entiredata collection if the shifted volume of homogeneity encloses all partsof the requested imaging volume. In other instances, active coil 110 maybe turned on and off in an interleaved manner if the requested imagingvolume spans both locations where the active coil 110 needs to be offand locations where active coil 110 needs to be on. In the illustrationof FIG. 2, when active coil 110 is energized, the main field homogeneitynear the top of the head may be sacrificed to enable the region ofuniformity to extend deeper into neck area 204. In some configurations,the extent of the shift is adjustable to accommodate the exact reachinto neck area 204. For example, the clinician, through the controlpanel on MRI system 100, may change the energization level of activecoil 110 to a particular level that corresponds to a degree of shiftinto neck area 204. The correspondence may be estimated beforehand andstored in a look-up table for access during an imaging session.

In the illustration of FIG. 1B, active coil 110 is housed within theenclosure for gradient coil 104. In this configuration, active coil 110may share the same active cooling with gradient coil 104. For example,both active coil 110 and gradient coil 104 may be cooled using the samecooling system circulates coolant such as a liquid helium of liquidnitrogen. This configuration may leverage an existing cooling systemthat already includes a reservoir for liquid coolant such as liquidnitrogen or liquid helium. The driving current for active coil 110 maybe in the range of 1-10 amperes while the gradient coil 104 may requiredriving current higher than 10 amperes. In other instances, active coil110 may be constructed on RF coil 106 and configured to be mountedinside gradient coil 104. In other instances, active coil 110 can behoused in the side wall 105 of the magnet and may require a separatecooling system. In the illustrations above, active coil 110 may coupleto a control unit on MRI system 100, for example, through poweramplifiers that provide the driving currents. The control unit may behoused in a separate control room away from the magnet. The control unitmay include processors or programming logic to configure the poweramplifiers that drive active coil 110.

FIG. 3 shows an example of a flow chart 300 for operating the activecoil 110 of MRI system 100. When the process flow initiates (302), agradient coil 104 may be operated to acquire magnetic resonance (MR)signals encoding a first MRI image over a first region inside a mainmagnet of the MRI system in which at least a portion of a subject isplaced, the first region being located within a volume of uniformmagnetic field with inhomogeneity below a defined threshold (304). Thedefined threshold may refer to a ceiling level of the delta of the mainmagnetic field over a span of distance, for example, 20 μT/25 cm for a0.5 T main magnet. The defined threshold may also refer to an upperbound of the spectral width of the FID signal from a desired volume. Forexample, the spectral width may be measured as the full width halfmaximum (FWHM) width. The operation may be initiated by an operatorconfiguring scanning parameters on a control panel. The gradient coilmay be coupled to a control unit of the MRI system 100 to receiveinstructions such that gradient waveforms are played accordingly toprovide field fluctuations to the main magnetic field. As noted earlier,gradient coil 104 may incorporate more than one subset of gradientcoils, each operating to effectuate field fluctuations in one spatialdirection inside inner bore 101 of MRI system 100. The first image mayreveal the subject's head area 202, with portions from neck area 204.When acquiring the MR signals encoding the first MRI image, shimmingcoil 109 may be used to compensate for susceptibility differenceswithin, for example, head area 202 and the portions from neck area 204.

Next, a determination may be made as to whether a new scan is required(306), for example, to reveal further details of neck area 204 inaddition to what has been portrayed on the first image, or to lookfurther into neck area 204. In some instances, the determination may bemade based on an input from an operator of MRI system 100. Otherinstances may incorporate an automatic region shifting aspect. Forexample, MRI system 100 may perform image recognition of the first MRIimage and determine where certain anatomical features in neck area 204are not fully captured and shifting the volume of uniform magnetic fieldis thus warranted. It may be determined the volume of uniform magneticfield covers the portions of interest from neck area 204 and there is noneed to look further, in which case, MRI system 100 may continueoperation without engaging active coil 110 (308).

When shifting is warranted, a control unit on MRI system 100 mayenergize an active coil to shift the volume of uniform magnetic fieldsuch that a second region inside the main magnet of the MRI system islocated within the shifted volume of uniform magnetic field, at least aportion of the second region being located outside of the volume ofuniform magnetic field before the volume of uniform magnetic field hasbeen shifted and at least a portion of the first region being locatedoutside of the shifted volume of uniform magnetic field after the volumeof uniform magnetic field has been shifted (310). The shifting may leavethe shifted volume of uniform magnetic field covering, for example, theportions of interest from neck area 204. As noted above, a particularenergization level can lead to a particular shift. Energizing activecoil 110 may involve running currents in the range of 1-10 amperes whileshimming coil 109 is associated with driving currents in the sub-ampererange, such as hundreds of milliamperes. Meanwhile, gradient coil 104may generally be driven by electrical currents in the 10-100 ampererange and may generally require active cooling via circulating coolants.

Once the volume of uniform magnetic field has been shifted to, forexample, cover portions of interest from neck area 204, a shim coil canbe operated to compensate susceptibility differences in the portion ofthe subject located within the shifted volume of uniform magnetic field(312). Once shimming is in place, gradient coil 104 may be operated toprovide field fluctuations to the main magnetic field such that MRsignals can be acquired that encode a second MRI image over the secondregion located within the shifted volume of uniform magnetic field(314).

As used herein, the terms “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

As used herein, the term “exemplary” means “serving as an example,instance, or illustration,” and should not be construed as preferred oradvantageous over other configurations disclosed herein.

As used herein, the terms “about” and “approximately” are meant to covervariations that may exist in the upper and lower limits of the ranges ofvalues, such as variations in properties, parameters, and dimensions. Inone non-limiting example, the terms “about” and “approximately” meanplus or minus 10 percent or less.

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover all modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

What is claimed is:
 1. A method for operating a magnetic resonanceimaging (MRI) system, the method comprising: operating gradient coilswhile turning off an active coil to acquire first magnetic resonance(MR) signals encoding a first MRI image with a first field of view, thefirst MR signals emitted from a first region inside a main magnet of theMRI system in which at least a portion of a subject is placed; andenergizing the active coil to shift the volume of magnetic field suchthat second MR signals are acquired from a second region inside the mainmagnet of the MRI system, wherein the first region is located within avolume of magnetic field while the second region is located within theshifted volume of magnetic field, and wherein the first region and thesecond region are both sufficiently homogeneous in field strength suchthat a field inhomogeneity of each region is within below a definedthreshold.
 2. The method of claim 1, further comprising: combining thefirst MRI image with a second MRI image reconstructed from the second MRsignals such that the first field of view is extended to include atleast a portion of the second region located outside the volume ofmagnetic field before the volume of magnetic field has been shifted. 3.The method of claim 1, further comprising: reconstructing a second MRIimage from the second MR signals acquired from the second region suchthat the second MRI image is generated with a reduced field of viewcompared to the first MRI image.
 4. The method of claim 1, furthercomprising: operating the gradient coils while turning off the activecoil to acquire third magnetic resonance (MR) signals encoding a thirdMRI image with a third field of view; and energizing the active coil toshift the volume of magnetic field such that fourth MR signals areacquired from a fourth region inside the main magnet of the MRI system.5. The method of claim 4, wherein the third field of view and the firstfield of view substantially overlap each other while the fourth field ofview and the second field of view substantially overlap each other. 6.The method of claim 1, wherein operating the gradient coils comprises:operating the gradient coils to generate gradient waveforms in tandemwith at least one radio-frequency (RF) pulse such that the first MRsignals include a spin-echo signal and the second MR signals alsoinclude a spin-echo signal.
 7. The method of claim 1, wherein energizingthe active coil comprises: energizing the active coil while operatingthe gradient coils to acquire the second MR signals.
 8. The method ofclaim 1, wherein energizing the active coil comprises: energizing theactive coil prior to operating the gradient coils to acquire the secondMR signals.
 9. The method of claim 1, further comprising: operating ashim coil while operating the gradient coils to compensatesusceptibility differences inside the portion of the subject locatedwithin the shifted volume of magnetic field with inhomogeneity below thedefined threshold.
 10. A magnetic resonance imaging (MRI) system,comprising: a housing having a bore in which at least a portion of asubject to be imaged is placed; a main magnet accommodated by thehousing and configured to generate a volume of magnetic field; an activecoil that when energized causes the volume of magnetic field to shift inlocation and transform in shape; pulse generating coils configured togenerate and apply radio frequency (RF) pulses in sequence to scan theportion of the subject; gradient coils configured to provideperturbations to the volume of magnetic field such that magneticresonance (MR) signals encoding an MRI image are acquired in response tothe applied RF pulses; and a control unit coupled to the MRI system andconfigured to: operate gradient coils while turning off an active coilto acquire first magnetic resonance (MR) signals encoding a first MRIimage with a first field of view, the first MR signals emitted from afirst region inside a main magnet of the MRI system in which at least aportion of a subject is placed; and energize the active coil to shiftthe volume of magnetic field such that second MR signals are acquiredfrom a second region inside the main magnet of the MRI system, whereinthe first region is located within the volume of magnetic field whilethe second region is located within the shifted volume of magneticfield, and wherein the first region and the second region are bothsufficiently homogeneous in field strength such that a fieldinhomogeneity of each region is within below a defined threshold. 11.The MRI system of claim 10, wherein the active coil and the gradientcoils are integrated into one mechanical assembly.
 12. The MRI system ofclaim 10, wherein the active coil is constructed as a removable moduleconfigured to be mounted along with a gradient assembly that houses thegradient coils.
 13. The MRI system of claim 10, wherein the active coilis constructed on an RF coil assembly configured to be mounted insidethe gradient coils.
 14. The MRI system of claim 10, further comprising:a group of shimming coils placed around the housing and configured tocompensate susceptibility differences inside the portion of the subjectlocated within the volume of magnetic field.
 15. The MRI system of claim10, wherein the control unit is further configured to: combine the firstMRI image with a second MRI image reconstructed from the second MRsignals such that the first field of view is extended to include atleast a portion of the second region located outside the volume ofmagnetic field before the volume of magnetic field has been shifted. 16.The MRI system of claim 10, wherein the control unit is furtherconfigured to: reconstruct a second MRI image from the second MR signalsacquired from the second region such that the second MRI image isgenerated with a reduced field of view compared to the first MRI image.17. The MRI system of claim 10, wherein the control unit is furtherconfigured to: operate the gradient coils while turning off the activecoil to acquire third magnetic resonance (MR) signals encoding a thirdMRI image with a third field of view; and energize the active coil toshift the volume of magnetic field such that fourth MR signals areacquired from a fourth region inside the main magnet of the MRI system.18. The MRI system of claim 17, wherein the third field of view and thefirst field of view substantially overlap each other while the fourthfield of view and the second field of view substantially overlap eachother.
 19. The MRI system of claim 17, wherein the control unit isfurther configured to: energize the active coil while operating thegradient coils to acquire the second MR signals.
 20. The MRI system ofclaim 17, wherein the control unit is further configured to: energizethe active coil prior to operating the gradient coils to acquire thesecond MR signals.